Bluetooth: samples/hci_uart: Add micro:bit configuration

The BBC micro:bit doesn't have UART HW flow control, so it needs to be
disabled to build for the board. The baudrate is also lowered to
115200 since 1Mbps is likely to be unreliable without flow control.

Change-Id: I1725a2a1e46cfbc0f57b4f5c4ee57ff52d9670e9
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>